In recent years, a brushless motor is used in a cooling fan commonly used for, for example, a refrigerator, because the brushless motor is long-life and meets a demand for energy efficiency. Such a cooling fan needs to be controlled to satisfy, for example, required airflows and driving sounds emitted in its surrounding area, which are preset values corresponding to air temperatures and temperatures of an object to be cooled by the cooling fan. It is known that variations in the airflows depend on variations in a rotational speed of a motor mounted to an impeller, and that the driving sounds depend on running torque of the motor.
Conventionally, a sensorless three-phase brushless motor controlled to be driven using pulse width modulation (hereinafter referred to as “PWM” as appropriate) is controlled by, for example, a brushless motor control device such as that described below. With the control device, a rotated position of the motor is first estimated from values of currents flowing through coils each in a different one of phases. An actual rotational speed of the motor is then calculated from an amount of change in the rotated position per unit of time. Then, according to the calculated actual rotational speed, a switching pulse width for a metal-oxide-semiconductor field-effect transistor (MOS-FET) device is controlled, the MOS-FET device mounted to an inverter circuit that enables the motor to be driven using PWM.
With this configuration, if there is an error in a current detection circuit for detecting values of currents flowing through the coils, an analog-digital (AD) converter circuit for enabling an output from the current detection circuit to be input to a microcomputer, or a clock generation circuit for generating an unit of time, an error also occurs in a calculated actual rotational speed and calculated torque, resulting in differences between the actual rotational speed and the torque and target values.
There has been proposed a conventional method for determining whether an error is occurring in a clock generation circuit, that is, the clock generation circuit is in an abnormal condition. The method includes, for example, inputting, to an RC filter, a pulse voltage generated based on a clock pulse at predetermined intervals, and determining that the clock generation circuit is in an abnormal condition if voltages at predetermined sampling timings fall outside a predetermined range (for example, see, PTL 1).
However, the conventional configuration, which can determine whether the clock generation circuit is in an abnormal condition, cannot determine whether a current detection circuit and an AD converter circuit are in an abnormal condition. Therefore, the method cannot prevent increased errors in a rotational speed and torque due to an abnormal condition of the AD converter circuit and the current detection circuit, and thus cannot prevent increased variations in an airflow of a cooling fan and a driving sound emitted by the cooling fan. To make allowance for the increased variations, a large motor is conventionally used for the cooling fan. The variations can also be addressed by adding a plurality of complex diagnostic circuits. This, however, requires a large microcomputer because a small microcomputer, which has fewer pins, cannot achieve a circuit configuration, resulting in increased cost.